The present invention generally relates to a spectrophotometer and, in particular, relates to a Fourier Transform Spectrophotometer wherein the energy sensitive area of the detector is fully illuminated regardless of the resolution setting.
In general, Fourier Transform (FT) Spectrophotometers are primarily utilized in the field of infrared analysis. Hence, such spectrophotometers are generally known as Fourier Transform Infrared (FT/IR) Spectrophotometers. The primary advantages of FT/IR spectrophotometers is that entire wavelength ranges can be analyzed faster, with more energy throughput and with reduced background stray light. Such advantages are well known in the spectrophotometer art and, consequently, have given rise to the increasing use of FT/IR spectrophotometers.
Conventionally, FT/IR spectrophotometers can be of either single beam or double beam variety. In a single beam instrument the sample is positioned in the path of the sampling light beam and subsequently removed or otherwise used to characterize a background signal. Thus, when the sample cell is removed or, for example, when the sample cell is filled with a reference fluid, the background noise signal which exists during a particular measurement can be ascertained and subsequently subtracted from the signal received when the sample is inserted in the path. Single beam instruments frequently require the disruption of the control environment in which the FT/IR functions to switch from a reference measurement to a sample measurement. Consequently, the noise measurement is neither simultaneous with the sample measurement nor is it performed under identical conditions.
As a means of avoiding this inaccuracy, double beam systems were developed and are well known in the art. In such systems, one beam is directed through the sample cell or sample material whereas a second beam, following a substantially identical optical path--except for that portion of the path passing through the sample--is introduced. Normally, this is accomplished by positioning, both before and after the sample cell, rotatable or otherwise movable mirrors, or other beam direction control optical elements. Accordingly, the incoming or incident beam directed to the sample cell is intermittently redirected along a path similar in length and other optical characteristics to that of the sample beam but which path does not contain sample. Thus, by optically subtracting the signal representative of the incident beam having passed through the sample from the beam having passed through the reference path, a more accurate measurement is achieved.
Although a double beam system, whether conventional or FT, is inherently more accurate and convenient than a single beam system, it nevertheless has a number of drawbacks when such a system is used for high sensitivity or very accurate measurements. One difficulty of the FT/IR spectrophotometers present known is that the sample beam and reference beam do not always impinge on the same area of the detector surface. This results because of the slight optical path differences caused by the presence of the sample in the sample beam path. As a result, the accuracy of the two measurements is reduced owing to the fact that different areas of the detector may produce slight differences in signal output.
This problem is particularly difficult when the area of a conventional Jacquinot stop is reduced in size to increase resolution. In addition, another image mismatch occurs at the beamsplitter of the interferometer of an FT/IR. Consequently, the beam division of the beamsplitter may be slightly different depending on the particular Jacquinot stop setting used. Such image mismatch, with respect to the beamsplitters, occurs primarily because the Jacquinot stop is focused on the detector and consequently the image thereof throughout the system, i.e., on the beamsplitter as well as various other points in the optical system, does not remain fixed when the resolution is varied via the Jacquinot stop.